Solar collectors include photovoltaics where sunlight is converted directly to electricity, solar thermal energy used to heat water, and large scale solar thermal power plants used to generate electricity. In these systems, solar energy is “collected” by placing panels or arrays of panels in the direct path of the sun. These panels are composed of mirrors or mirror-like material to reflect solar energy to a specific point for collection, or are made up of a variety of absorbent materials. Systems where absorbent materials are used can further be divided into systems where solar energy is collected in cells or where solar energy is absorbed as thermal energy to heat either water or a heat-transfer fluid, such as a water-glycol antifreeze mixture. Most commercially available solar cells are made from wafers of very pure monocrystalline or polycrystalline silicon. Such solar cells typically can attain efficiencies of up to 18% in commercial manufacture. The silicon wafers used to make them are relatively expensive, making up 20-40% of the final module cost. The alternative to these “bulk silicon” technologies is to deposit a thin layer of semiconductor onto a supporting material such as glass. Various materials can be used such as cadmium telluride, copper-indium-diselenide and silicon. There are basically three types of thermal collectors: flat-plate, evacuated-tube, and concentrating. A flat-plate collector, the most common type, is an insulated, weatherproofed box containing a dark absorber plate under one or more transparent or translucent covers. Evacuated-tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer tube and an inner tube, or absorber, covered with a selective coating that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn (“evacuated”) from the space between the tubes to form a vacuum, which eliminates conductive and convective heat loss. Concentrating collector applications are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid.
Most existing solar photovoltaic power generating systems generally do not use a primary concentrator because concentrating the sunlight would increase the temperature of the photovoltaic cell and significantly reduce its efficiency of conversion. A solution to this problem is to use a cooling system for the photovoltaic cell. But such a solution is considered expensive if low efficiency, inexpensive, solar photovoltaic cells are used. Another solution is to use more efficient, but more expensive, solar photovoltaic cells while reducing the area of solar photovoltaic cells required to generate a desired power output by using one stage of concentration. Depending on the amount of concentration achieved, this would lower the cost for the solar photovoltaic cells significantly while allowing for the added cost of the cooling system. An added benefit for the increase in illumination is a further increase in the efficiency of conversion of the solar photovoltaic cell.
Existing solar thermal power generating systems that are used to generate electricity generally already use one stage of concentration of the solar energy. That is, solar energy is focused by a collector directly onto a receiver. The concentration of the solar energy at the receiver allows for elevated temperature at the receiver analogous to a magnifying glass. The smaller the area into which the light is focused in a one-stage concentrator, the higher the temperature that the receiver can reach and the higher the apparent efficiency. However, the volume of the receiver must be fixed so that the receiver has a sufficiently sized reservoir to generate the designed power from the system. This, in turn, means that only a small part of the surface area of the receiver is heated from the energy of the sun while the remainder of the area loses energy. This leads to a decline in effective efficiency, as the hot reservoir of the system cannot reach an equilibrium temperature that is the same as that reached by the smaller illuminated area of the receiver. It is necessary to distribute the energy symmetrically over the receiver as well as simultaneously focusing the energy. The temperature of the reservoir of the receiver determines the efficiency of the system and ultimately, the power that can be extracted from the solar energy collected. The temperature of the reservoir as a whole must be elevated if additional power is to be generated by the system above that originally specified.
Accordingly, there is a need for solar concentrators that provide high efficiency of concentrations and conversion of solar energy.